1. Field of the Invention
The present invention relates to a method for producing an image. More particularly, it relates to a method for producing a heat resistant and durable image having high resolving power and high image contrast.
2. Description of the Prior Art
The optical density of a silver image formed in the emulsion layer of a photographic material which comprises a silver halide emulsion layer coated on a support by exposing and developing the photographic material gradually decreases from its maximum value to a background value at the edge of the silver image. The spacing between the maximum image density portion and the background is usually about one micron. Therefore, it is difficult to obtain a high contrast silver image having closely separated (about one micron) lines or spacings. Silver grains existing between adjacent image lines reduce the image contrast and resolving power.
Moreover, since such an emulsion layer is colored due to thermal decomposition of the binder when heated to about 150.degree. to 200.degree. C., it cannot be used for purposes requiring heat resistance.
One field which requires a heat resistant image is "super-microphotography". An image reduced to 35 mm film size from a 9 by 14 inch (23 by 36 cm) size original with a reduction ratio of about 10 is usually called a "microphotograph," and an image further reduced (to about 2 by 3 mm) by a factor of about 10 is called a "supermicrophotograph." A microphotograph can thus be considered to be an image reduced by a factor of about 10 and a super-microphotograph an image reduced by a factor of about 100.
Since the image size of a super-microphotograph is about 2 by 3 mm or smaller, the enlarging factor is about 100 (10,000 based on area ratio) when a supermicrophotograph is projected on a screen to provide the original image size. Consequently, a light intensity of about 10 million lux is necessary on the image surface of the super-microphotograph if the image projected on a transmission type screen, e.g., with a blackened back surface, is to have a light intensity of about 100 lux when the screen has a transmission optical density of 1. In fact, the super-microphotograph is illuminated with a light intensity of about 12 to 13 million lux to compensate for the loss of the projection lens. The temperature of the emulsion layer of the super-microphotograph increases to several hundred degrees C., due to the heat generated by the light absorbed in the emulsion layer, when it is continuously illuminated with such a strong light. As a result, the binder of the emulsion layer is thermally decomposed and colored to cause the image projected on the screen to be dim and colored. Since the silver image areas absorb light well, the temperature of these areas preferentially increases and the binder of these areas is preferentially decomposed, whereafter the decomposition spreads into the surrounding areas. Decomposition of the binder in even the non-silver image areas proceeds in an accelerated manner once it is slightly colored and light absorption occurs.
Heretofore, emulsion masks and hard surface masks have generally been used as photomasks in microelectronic manufacturing processes. However, an emulsion mask has low edge contrast, as described above, and such low mechanical strength that it is easily damaged, that is, durability is poor. On the other hand, a hard mask is quite durable, but the process for production thereof is complicated. Also, the production of a had mask requires a photoetching process that uses a photoresist which has low sensitivity and requires long exposure times.
Another method for producing a hard surface mask is described in U.S. Pat. No. 3,567,447 which comprises exposing and developing a photographic material which comprises a silver halide emulsion layer coated on a glass support to form a silver image, heating the photographic material to about 400.degree. C., selectively removing the emulsion layer at the non-silver image areas with a gelatin removing solution to uncover the glass support lying thereunder, depositing a metal (e.g., chromium) layer over the entire surface of the photographic material, removing the metal layer at the silver image areas with a metal removing solution, and then removing the emulsion layer at the silver image areas. In this method, the metal layer is deposited on the glass support after a relief image consisting of gelatin binder and silver grains is formed, and, accordingly, procedures for depositing the metal layer are limited (for example, the glass support cannot be treated with a strong acid or alkali to increase adhesion between the metal layer and the glass support since the binder is attacked by a strong acid or alkali). Further, this method is complicated and difficult to practice. Moreover, the abovedescribed metal removing solution may attack the metal layer at non-silver image areas and decrease the durability of the finally obtained metal mask (for example, when aluminum is used as the metal layer and sodium hypochlorite is used to remove the aluminum layer at the silver image areas, not only the aluminum layer at the silver image areas but also the aluminum layer at the non-silver image areas is removed, since sodium hypochlorite is an etchant for aluminum). Furthermore, the smoothness of the edges of a photomask obtained by this method is poor.